Highly rated internal combustion engines have conventionally used crankshaft bearings comprising either a copper-based alloy or an aluminium-based bearing alloy bonded in some manner to a strong backing or substrate material such as steel, for example. The actual working surface of the bearing alloy, i.e. that surface which faces the engine crankshaft journal surface has also often been provided with a so-called overlay coating which is a thin coating of a relatively softer metal alloy such as lead-tin, lead-tin-copper or lead-indium for example. The purpose of the overlay coating is to provide conformability and dirt embeddability properties to the bearing. Conformability is that property of a bearing which allows it to accommodate slight mechanical misalignments between the bearing and shaft surfaces and is a measure of the ability of the overlay alloy to distribute the applied load. Dirt embeddability is that property which allows debris particles in the lubricating oil to be embedded in the soft overlay alloy without causing damage such as scoring of the shaft. Whilst overlay coated bearings have some technical advantages they have the significant disadvantage of being expensive to make due to the overlay generally being deposited by electroplating or sputter deposition which are relatively very labour intensive processes.
Manufacturers of motor vehicles are more frequently asking for bearings which do not have overlay coatings as they are cheaper to buy. However, some engines whilst not possessing a particularly high specific power output, due to their design, impose high loads on the crankshaft bearings or possess particularly thin oil films between the bearing and shaft journal and are consequently prone to so-called “scuffing” of the bearing surface. Scuffing is where metal to metal contact between the crankshaft journal surface and the bearing surface occurs, i.e. the oil film at the point of contact is ruptured allowing metal to metal contact. Scuffing relates to momentary metal to metal contact without actual seizure and consequent failure of the bearing. However, whilst overlay coated bearings are especially scuff resistant, most of the conventional copper and aluminium based alloys are relatively poor in terms of scuff resistance when used without an overlay. The ability to withstand scuffing is a measure of the conformability of the alloy. In contrast to scuffing, seizure is related to lack of compatibility of the alloy and generally causes failure of the bearing (and possibly of the engine) when it occurs.
GB 2 358 872, of common ownership herewith, describes aluminium-based bearing alloys for sliding bearings, principally for hydrodynamically lubricated engine bearings, comprising in weight % 5-10 tin; 0.7-1.3 copper; 0.7-1.3 nickel; 1.5-3.5 silicon; 0.1-0.3 vanadium; 0.1-0.3 manganese; balance aluminium apart from incidental impurities. This bearing alloy when bonded to a strong backing material such as steel, for example, has good fatigue strength and also exhibits superior scuff resistance when used without an overlay coating as described above and compared with other known high-strength aluminium-based bearing materials.
However, more recent developments in high output engines such as, for example, relatively small, high-revving turbo-charged diesel engines, whilst still requiring high resistance to scuffing and seizure also now require increased resistance to fatigue and wear, i.e. higher strength. However, the cost imperative is still present which precludes the use of overlay coatings. Usually, the two requirements of increased fatigue strength and maintained or increased scuffing and/or seizure resistance are mutually incompatible; higher strength usually resulting in a reduction of scuff/seizure resistance.
We have now found a formulation for an aluminium alloy-based bearing material which surprisingly satisfies the hitherto mutually incompatible requirements of increased strength and increased seizure resistance.